1. Field of the Invention
The present invention relates to a brushless motor that has a skew structure. More particularly, the invention relates to a brushless motor having a step-skew structure by means of segment magnets.
2. Related Art Statement
Hitherto known, as means for reducing cogging torque and torque ripple, is the skew structure in which the rotor poles or the like are inclined to the axis direction. In most brushless motors of skew structure, ring magnets are used as pole magnets. In any motor using ring magnets, the magnets are skew-magnetized in order to reduce cogging torque and torque ripple.
In brushless motors designed for use in electric power steering apparatuses, the segment magnets that can be magnetized at high flux density are increasingly used as rotor magnets, thereby to meet the demand that the motors should be smaller and should yet achieve a large output. However, the segment magnets (right-angled magnetic field type magnets) cannot be skew-magnetized due to the restriction imposed on its manufacture. In any motor that has segment magnets, the segment magnets are therefore stacked one on another, thus achieving a so-called step skew in order to perform skew structure.
In the motor of step-skew structure, pluralities of segment magnets are arranged in the axial direction. In each stage, each segment magnet is displaced by a prescribed angle in the circumferential direction. The magnetic poles are thereby displaced stepwise in the axial direction, forming a step-skew structure. Each magnet is secured to the outer circumferential surface of the rotor core, by using an adhesive. The adhesive of thermosetting type is used in most cases. Each magnet to which the adhesive is applied is adhered on the rotor core. On the rotor core, the magnets are positioned by using a dedicated jig and they are heated, whereby the magnets are fixed on the rotor core.
In the motor of such a step-skew structure, the more the magnets are stacked, the lower the steps will be, thus the cogging torque and torque ripple can be reduced. However, as the number of magnets stacked increases, the sum of the positioning errors of magnets will increase and the variation from the design specification in terms of skew angle will increase. Consequently, the motor of the step-skew structure fails to obtain performance as designed. Particularly, if secured to the rotor core with the adhesive, the magnets are easily displaced during the manufacture of the motor. Inevitably, the magnets are likely to have positional errors. Further, because adhesive is applied to secure the magnets to the rotor core, the excessive adhesive may stick to the jig, which lowers the working efficiency of the motor.